Task 2: Key Concepts about Identifying Important Food Group Sources of
Nutrients

There are two different ways to consider food
sources of nutrients—as “important” vs. “rich” sources.
Rich
sources are those foods with the greatest concentration of a
nutrient; important sources are those that contribute the most to a
population’s
dietary intake. For example, sardines are a rich source of calcium,
but they are not a very important source in the US diet, because
they are consumed relatively infrequently. Fluid milk, on the other
hand, is both a rich and important source of calcium because it
contains high levels of calcium (rich) and it is consumed frequently
in the population (important). A food composition table or database can
provide information about rich sources of nutrients, whereas
population intake as well as food composition data are needed to
identify important sources. This task demonstrates the latter.

Food sources of nutrients are identified using
the 24-hour recall data, because these data provide the necessary
detail regarding all foods consumed and contain nutrient values
associated with each food.

Ratio of Means

Identifying food sources of nutrients involves ratios,
because it deals with the proportion of a given nutrient that is supplied by a
given food. More specifically, it involves a ratio of means, because the
percentage contribution of each food to the population’s total consumption is of
most interest.

A ratio of means provides a single aggregate value for the
whole population, so individual values are not available for each person.
However, if subgroup differences are of interest then separate ratios of means
can be estimated for each group.

When data from one or two 24-hour recalls are used to
estimate a ratio of means, the mean in the numerator and the mean in the
denominator can each be considered an estimate of usual intake (for reasons
stated in Task 1 of “Module 14: Estimate Population Mean Intakes”). Therefore,
no specific statistical adjustments are necessary.

Grouping Foods for Analysis

Identifying important food sources of nutrients requires
grouping the foods reported in the survey using either pre-existing schemes or
one developed for the purpose. It is important to list all the food groups used
in the analysis when reporting results, because decisions regarding how the
foods are grouped can have a major influence on the relative contributions of
one food group vs. another. For information about some pre-defined food
grouping schemes, see “Module 4: Resources for Dietary Data Analysis” of the Survey Orientation Course.

IMPORTANT NOTE

In identifying important food sources of nutrients, use the ratio of means and be clear about how foods are grouped.